1. The Field of the Invention
This invention relates to the field of optics, and more particularly, to the use of optically induced changes in the refractive index of a medium for use as an optical switch.
2. The Prior Art
The importance of optics in many fields of technology has rapidly increased in the recent past and continues to increase at an ever faster pace. For example, the use of optical fibers in communications is becoming increasingly accepted and their use is expanding exponentially. In addition, the use of optical devices such as lasers is increasing in fields as diverse as medicine, defense, and communications.
With the increasing use of optical devices comes an increasing need for methods and apparatus for controlling the path of the light used in such devices. The basic types of devices for controlling the path of the light are well known and may be generally thought of as including conventional mirrors and lenses.
As the uses for optical devices increases, the need for efficient methods for controlling the path of light, particularly in the form of optical switches, also becomes more critical. Most conventional optical switches involve the use of mirrors and lenses positioned in such a manner as to provide the desired switching of a light beam. These mirrors and lenses may be controlled by some method for changing their position, such as a motor. As a result the mirror or lens may be moved into or out of the light paths and the angle at which the light beam intersects the mirror or lens may be varied.
Several switching devices of the type discussed above have been developed and are well-known in the art. Many of these devices are used in switching light beams carried by optical fiber communication systems. It is often critical for light carried by one optical fiber to be transferred to another optical fiber so that the desired communication link may be established.
In one such device, two quarter-period graded refractive index ("GRIN") lenses are aligned so that one surface of each lens abuts the other lens. On the surface of the first lens which does not abut the second lens are one or more input optical fibers. On the opposite surface of the second lens are a plurality of output fibers. The two lenses are mounted so that they are capable of being turned with respect to one another.
In order to switch an input beam to a different output fiber, the lenses of such prior art devices are simply rotated with respect to one another until the input beam is directed toward the desired output fiber. Thus, this type of device may be constructed by combining conventional GRIN lenses and mechanical devices in order to achieve the desired beam switching.
Another approach to beam switching, which also employs conventional technology, combines lenses and mirrors, or other reflecting surfaces. In this type of device a plurality of optical fibers are connected to one surface of a GRIN lens. On the opposite end of the lens, however, is a movable reflecting surface. Therefore, light passing through the optical fibers into the lens will strike the reflecting surface and then be reflected back through the lens. If positioned correctly, the reflecting surface can direct a light beam into the desired optical fiber. By changing the position of the reflecting surface, the reflecting surface can switch a light beam from fiber to fiber as desired.
An additional attempt by others to provide a usable optical switch involves the use of a transparent sheet of material with a relatively small area covered with opaque reflecting material. Two perpendicular beams are passed through the reflecting sheet. The material is positioned at an angle with respect to the beams such that when the material is in a first position, the first beam passes through the material to a receptor. When the material is moved to a second position, the first beam is blocked from the receptor by the reflecting surface. However, the second beam strikes the opposite side of the reflecting surface and is directed to the receptor. Thus, by moving the material and thus changing the position of the reflecting surface, it is possible to select whether the first or second beam reaches the receptor. Again, conventional mechanical and optical devices are combined in this device to provide an optical switch.
Recently, somewhat more exotic switching devices have been developed using more advanced physical concepts. For example, certain devices have been developed which take advantage of the unique physical properties of the interface between a linear and an optically non-linear material. A non-linear material is generally one whose refractive index changes with changes in the intensity of light impacting the material.
For example, an input light beam may be propagated in a linear material with a particular intensity and angle of incidence on the interface between the linear and non-linear material such that the beam is totally reflected at the interface. Since the reflectivity of the interface is intensity dependent, however, if the intensity of the input beam is increased at the point it strikes the linear-nonlinear interface, at some particular intensity the beam will be only partially reflected. The remaining portion of the beam will cross the interface into the non-linear medium.
The same effect can be accomplished by directing a second "control beam" at the point where the input beam contacts the interface such that the combined intensities of the control beam and the input beam are sufficient to cause a portion of the input beam to pass into the non-linear medium.
Another application of the phenomena of intensity-dependent reflectivity is in the construction of a switch used to switch an infrared signal laser. In this device, an infrared laser is directed at a semiconductor material. The semiconductor material is chosen so that it is ordinarily transparent to the laser beam. Added to the system is a "control beam" which is selected so that it is sufficiently high frequency to produce free carriers in the semiconductor. When sufficient free carriers are produced within the semiconductor, the semiconductor becomes opaque to the signal laser beam. As a result, instead of propagating through the semiconductor, the signal laser beam is reflected or switched.
The switches described above are all dependent on changes in the reflectivity or degree of transparency at an interface between two materials. As mentioned above, these changes are the result of the photorefractive effect where one of the materials has a non-linear refractive index. Hence, these devices operate on the principle that no light is transmitted through an interface between two dissimilar materials when the angle of light ray incidence is greater than that of the critical angle. The critical angle is a function of the ratio of the refractive indexes of the two materials.
These switches work by changing the refractive index of one of the materials thereby changing the critical angle. If this new critical angle is greater than the angle of incidence of the controlled beam, then transmission occurs with a subsequent decrease in reflection. The refractive index of one of the materials is changed by the photorefractive effect and thus constitutes a light activated light valve or switch. The effects described above are due generally to critical angle changes as a result of the photorefractive effect.
As an additional point, it is important to note that all of these devices are dependent on "localized" effects. The materials used in these devices are modified in their optical properties or at a localized point, and not throughout the material.
The need for efficient high speed optical switching devices is clear. As mentioned above, the use of optical fiber communications systems in now widely accepted and is expected to increase in importance. Since optical fiber communications are fast, traveling at the speed of light through the subject medium, the speed of the switching device used is likely to be the factor which limits the speed at which the system can function.
While numerous devices have been constructed which combine optical and mechanical elements to form a switch, a motor of some type is generally used to move a lens or mirror such that the direction of a subject light beam can be changed. The time that it takes for such a motor to operate, however, will be high in comparison to the speed at which the light travels through the system. As a result, the use of conventional motors coupled with conventional lenses will produce a relatively slow system.
In addition, it is critical that the switch not significantly distort the switched signal. Excessive distortion will cause the signal to be degraded and possibly lose its usefulness. Thus, switches are required which are not only quick but are also efficient in that they do not produce signal noise.
It will be appreciated that the use of light beams coupled with high speed efficient switches may have application in an optical computer. The importance of optical switches in the construction of a computer resides in the speed with which those switches may potentially operate and the capacity of such switches to process information. Because of physical considerations, switching times within current electronic switches are generally on the order of nanoseconds (10.sup.-9). Switching times in electronic switches are not expected to increase significantly because of the physical limitations.
In the event a switch could be produced which was totally optical and did not require electronic or mechanical components, switching times would be expected to be on the order of picoseconds (10.sup.-12). This would be approximately 1000 times faster than the switching times of conventional electronic switches. Thus, if an acceptable optical switch could be constructed for use in a computer, it appears feasible that such a computer could be approximately 1,000 times faster than conventional computers.
An additional advantage of the optical switch is the potential for such a switch to process more than one signal at a time. Since there is no particular interference between parallel beams of light, it may be possible for two or more distict signals to be processed simultaneously. Conventional electronic devices are obviously not able to process multiple streams of electrons simultaneously because of the electrical and magnetic interaction among electrons.
As previously mentioned, one desirable feature of the use of optics generally is the fact that there is no magnetic or electrical interference or distortion. Light beams are not subject to changes in electrical fields since they are composed of photons rather than electrons. As a result, a switch which could operate independent of electric or mechanical devices would be a significant improvement in the art. In addition, it appears highly probable that if such a switch could be constructed, response times in the order of picoseconds would be possible.
It is apparent that what is needed in the art are methods and apparatus for constructing an optical switch capable of operation independently of electrical and mechanical devices and which does not rely on localized changes in refractive index as described by Snell's law. It would be a major advancement in the art if such a switch could be constructed which had picosecond response times.
It would be a further advancement in the art if such a switch could be constructed which could employ solids, liquids, or gases as switching media. It would also be an advancement in the art if such a switch could be constructed which was capable of switching multiple parallel beams. It would be a still further advancement in the art if an optical switch could be constructed which was capable of switching a high power light beam using a relatively low power light beam, thereby effectively constituting an amplifier in the light beam.
Such inventions capable of achieving these advances over the prior art devices are disclosed and claimed herein.